Currently, an electrical water heater commonly available in the market usually comprises an electrical heating wire, an inner container and a temperature controller etc. During actual use, the electrical water heater has to heat up the water from time to time to maintain water temperature. Accordingly, more electricity will be wasted. Also, the time required for heating is long and there are deeper concerns about the heater's safety during its actual use. Besides, the existing electrical water heater is relatively large in size and requires more spaces for installation.